Consistent Inter-individual Differences Research Articles

Predation Risk Experienced by Tadpoles Shapes Personalities Before but Not After Metamorphosis.

Consistent inter-individual differences in behaviour, that is, personalities, can emerge as a result of inter-individual differences in ontogenetic experience, and predation risk is a potent one. As personalities develop over lifetime, however, they may also be broken by ontogenetic transitions of the individual. Here we first tested the hypothesis that consistent inter-individual differences in larval behaviour arise under predation challenge, and are entangled with differences in body size. We then tested the hypothesis that adult behavioural type is related to body size rather than to larval behavioural phenotype. To test these hypotheses, we performed a longitudinal study following the development of about 50 moor frogs, Rana arvalis. We manipulated their larval and current environment, and recorded their behaviours repeatedly, under control conditions, invertebrate predators' chemical cues or in live predator presence. Partially in line with our predictions, the ontogenetic experience of predator presence led to personality emergence in tadpoles, yet their behaviour was not explained by their body size. This pattern was lost over metamorphosis. According to predictions, pre-adult moor frog behaviour was affected by their body size-time to exit shelter was shorter in larger frogs-but neither by their behaviour as tadpoles nor by their larval environment, that is, tadpole predator-exposure experience. Our results show that individual behavioural tendencies can be well decoupled between prior and post metamorphosis, which adds to the growing empirical evidence supporting adaptive decoupling hypothesis.

Personality matters – The interplay between consistent individual differences and mouse welfare in female C57BL6/J mice

To ensure good welfare of animals in human hands, it is essential to modify housing conditions according to the animals’ needs. Traditionally, the effects of such modifications are studied by means of group-level comparisons, thereby widely neglecting consistent inter-individual differences (i.e., so-called ‘animal personalities’). However, as animals with distinct personality types might differ in their environmental needs and hence react differently to the same environment, such systematic inter-individual differences might have important welfare consequences. This becomes particularly apparent under laboratory conditions, where animals are typically housed under highly standardized and barren environments. Against this background, we here aim to investigate personality-dependent welfare consequences in response to different housing conditions in laboratory mice. Female C57BL/6J mice were characterized for their personality type in exploration behavior and the most and the least explorative individuals were set up in either simple or in highly complex housing conditions that included constantly changing environmental enrichment items. We monitored individual welfare by studying behavioral, physiological, and immunological outcome measures. Besides personality-dependent differences in immune parameters and overall improved welfare under complex housing conditions, we indeed found hints that individual mice were differently affected in their welfare depending on the specific combination of personality type and housing condition. Specifically, highly explorative mice appeared to be more adversely affected by simple housing, but also profited more from complex housing compared to low explorative mice. These findings indicate that welfare promoting adjustments do not necessarily benefit all individuals equally and therefore, call for a shift of perspectives in the evaluation of animal welfare.

How to explore a new environment: exploratory tactics of the black rat (Rattus rattus).

The black rat (Rattus rattus) is a unique model for studying exploratory tactics due to its enormous colonizing potential. Considerable behavioral variability and consistent interindividual differences might help populations inhabit new environments and persist there even under intense pressure. Additionally, the affinity of the black rat for climbing might be another advantage, widening their potential niche. In this study, we describe the exploratory tactics of the black rats when introduced to a novel environment. In the first experiment, we tested 12 rats and calculated repeatability of their behaviors across 12 sessions of an enriched open-field test. We concluded that climbing is a highly repeatable behavior that serves as an important source of interindividual variability. In the second experiment, we tested 24 black rats in a unique L-shaped arena. Each rat was tested twice. We found that the majority of rats distributed their activity evenly, exploring each part of the apparatus for a similar amount of time, thus maximizing their chances of finding resources. Nevertheless, these "even" explorers still greatly differed in their level of activity, orderliness and affinity for climbing, generating large variability. In contrast, the minority of rats concentrated their activity only on a section of the new environment and were therefore characterized as selective explorers. Overall, we concluded that a combination of such exploratory tactics as well as a bias for even explorers enables black rats to quickly colonize new environments and persist there even under unfavorable conditions.

Individual differences in human gaze behavior generalize from faces to objects

Individuals differ in where they fixate on a face, with some looking closer to the eyes while others prefer the mouth region. These individual biases are highly robust, generalize from the lab to the outside world, and have been associated with social cognition and associated disorders. However, it is unclear, whether these biases are specific to faces or influenced by domain-general mechanisms of vision. Here, we juxtaposed these hypotheses by testing whether individual face fixation biases generalize to inanimate objects. We analyzed >1.8 million fixations toward faces and objects in complex natural scenes from 405 participants tested in multiple labs. Consistent interindividual differences in fixation positions were highly inter-correlated across faces and objects in all samples. Observers who fixated closer to the eye region also fixated higher on inanimate objects and vice versa. Furthermore, the inter-individual spread of fixation positions scaled with target size in precisely the same, non-linear manner for faces and objects. These findings contradict a purely domain-specific account of individual face gaze. Instead, they suggest significant domain-general contributions to the individual way we look at faces, a finding with potential relevance for basic vision, face perception, social cognition, and associated clinical conditions.

Personality variation is eroded by simple social behaviours in collective foragers.

The movement of groups can be heavily influenced by 'leader' individuals who differ from the others in some way. A major source of differences between individuals is the repeatability and consistency of their behaviour, commonly considered as their 'personality', which can influence both position within a group as well as the tendency to lead. However, links between personality and behaviour may also depend upon the immediate social environment of the individual; individuals who behave consistently in one way when alone may not express the same behaviour socially, when they may be conforming with the behaviour of others. Experimental evidence shows that personality differences can be eroded in social situations, but there is currently a lack of theory to identify the conditions where we would expect personality to be suppressed. Here, we develop a simple individual-based framework considering a small group of individuals with differing tendencies to perform risky behaviours when travelling away from a safe home site towards a foraging site, and compare the group behaviours when the individuals follow differing rules for aggregation behaviour determining how much attention they pay to the actions of their fellow group-members. We find that if individuals pay attention to the other members of the group, the group will tend to remain at the safe site for longer, but then travel faster towards the foraging site. This demonstrates that simple social behaviours can result in the repression of consistent inter-individual differences in behaviour, giving the first theoretical consideration of the social mechanisms behind personality suppression.

Intra-individual variation of hen movements is associated with later keel bone fractures in a quasi-commercial aviary

Measuring intra- and inter-individual variation in movement can bring important insights into the fundamental ecology of animals and their welfare. Although previous studies identified consistent differences in movements of laying hens within commercial aviaries, the level of consistency was not quantified, limiting our capacity to understand the importance of individual movements for welfare. We aimed to quantify the scope of intra- and inter-individual differences in movements of commercial laying hens and examined their associations with indicators of welfare at the end of production. We quantified individual differences in one composite daily movement score for 80 hens over 54 days post-transfer to a quasi-commercial aviary. Results showed consistent inter-individual differences in movement averages, explaining 44% of the variation, as well as individual variation in predictability and temporal plasticity (at the population-level, hens increased their movements for 39 days). Hens that were more predictable in their daily movements had more severe keel bone fractures at the end of production while we found no such correlation between daily movement averages (individual intercept) and welfare indicators. Our findings highlight the importance of inter-individual difference in intra-individual variation of movements to improve poultry welfare.

Age, but not hand preference, is related to personality traits in common marmosets (Callithrix jacchus).

The proximate mechanisms underlying animal personalities, i.e. consistent inter-individual differences in behaviour, are a matter of discussion. Brain lateralization, expressed as the preferred use of the contralateral limb, has been suggested as one of these mechanisms. In this study, we measured a proxy of brain lateralization in captive common marmosets (N = 28) by testing hand preference in a simple food-reaching task and evaluated personality by coding a wide range of behaviours observed in daily situations. We explored the links between personality and both direction and strength of hand preference, as well as age and sex, using linear models. Principal component analysis revealed that the stable behavioural variables were organized in three personality dimensions: Agreeableness, Extraversion and Neuroticism. Regarding hand preference, 14 individuals were left-handed, seven were right-handed and seven were ambilateral. Contrary to our predictions, we did not find any relationship between personality scores and hand preference or sex. Instead, age was a significant predictor of personality scores, with older individuals being more agreeable and less extraverted. The link between brain lateralization and personality seems to be equivocal and dependent on personality and brain lateralization assessment methods. Further examinations of other proximate mechanisms, such as physiology or (epi)genetics, may elucidate what drives personality variation in common marmosets.

Environment-dependent benefits of interindividual variation in honey bee recruitment

Interindividual differences in behaviour within the members of a social group can affect the group's productivity. In eusocial insects, individual differences between workers in a colony play a central role in division of labour and task allocation. Extensive empirical and theoretical work has highlighted variation in response thresholds as a proximate mechanism underlying individual behavioural differences and hence division of labour. However, other response parameters, like response probability and intensity, can also affect these differences. In this study, we first extended a previously published agent-based model on honey bee, Apis mellifera , foraging to understand the relative importance of response (dance) probability and response (dance) intensity in the task of recruitment. Comparing variation obtained from the simulations with previously published empirical data, we found that response intensity played a more important role than probability in producing consistent interindividual differences in recruitment behaviour. We then explored the benefits provided by this individual variation in recruitment behaviour to the colony's collective foraging effort under different environmental conditions. We found that individual variation led to a greater energy yield per forager, but only when food was abundant. Our study highlights the need to consider all response parameters while studying division of labour and adds to the growing body of evidence linking individual variation in behavioural responses to the success of social groups. • A. mellifera foragers show consistent individual differences in recruitment. • Agent-based models were used to explore the role of these differences in the colony. • Empirical findings were reproduced by including response probability and intensity. • Individual variation reduced total recruitment and slightly increased energy yield. • Studies on division of labour should include thresholds, probability, and intensity.

Behavioral variation across the days and lives of honey bees

SummaryIn honey bee colonies, workers generally change tasks with age (from brood care, to nest work, to foraging). While these trends are well established, our understanding of how individuals distribute tasks during a day, and how individuals differ in their lifetime behavioral trajectories, is limited. Here, we use automated tracking to obtain long-term data on 4,100+ bees tracked continuously at 3 Hz, across an entire summer, and use behavioral metrics to compare behavior at different timescales. Considering single days, we describe how bees differ in space use, detection, and movement. Analyzing the behavior exhibited across their entire lives, we find consistent inter-individual differences in the movement characteristics of individuals. Bees also differ in how quickly they transition through behavioral space to ultimately become foragers, with fast-transitioning bees living the shortest lives. Our analysis framework provides a quantitative approach to describe individual behavioral variation within a colony from single days to entire lifetimes.

Familiarity, personality, and foraging performance in three-spined sticklebacks

Animals can gain large benefits from living in groups but must coordinate with their groupmates in order to do so. Social interactions between groupmates drive overall group coordination and are influenced by the characteristics of individual group members. In particular, consistent inter-individual differences in behaviour (e.g. boldness) and familiarity between individuals in groups profoundly affect the individual interactions that mediate group coordination. However, the effects of boldness and familiarity have mostly been studied in isolation. Here we describe how familiarity and boldness interact to affect individual performance, leadership, and group coordination in small shoals of three-spined sticklebacks (Gasterosteus aculeatus) solving a novel foraging task. Groups of higher average boldness were less cohesive, but only when group members were familiar with one another. Familiarity affected shy and bold individuals’ foraging performance and leadership tendencies differently depending on group characteristics: the shyest group member experienced declining foraging success and leadership with increased group boldness in familiar groups, but experienced the opposite effect on foraging and no effect on leadership in unfamiliar groups. The boldest group member, in contrast, exhibited the opposite pattern: leading and eating more with increasing group boldness in familiar groups, but eating less with increasing group boldness in unfamiliar groups. These results suggest that both boldness and familiarity are important for establishing group behaviour and coordination, and that consistent inter-individual differences in behaviour may primarily impact group coordination once familiarity has been established.

Personality assessment of headstart Texas horned lizards (Phrynosoma cornutum) in human care prior to release

The study of animal personality is a growing field that has applications for welfare of animals living in captive settings. We measured personality traits (activity, exploration, and neophobia) in Texas horned lizards (Phrynosoma cornutum) living in human care before they were released to their natal habitat as part of a headstart program. We found evidence of consistent inter-individual differences in activity and exploration, but not neophobia. We also identified a positive correlation between activity and exploration, such that more active lizards were also more likely to explore a novel environment. These results suggest that Texas horned lizards have individual differences in response to their environment, which can inform husbandry decisions. Extensions of this work could also have implications for conservation of Texas horned lizards and for headstart programs focused on reptiles.

Personality as a Predictor of Time-Activity Budget in Lion-Tailed Macaques (Macaca silenus).

Simple SummaryTime-activity budgets describe how animals divide their day into various behaviours and activities, e.g., time spent foraging or resting. Activity budgets can serve as crucial indicators of energy intake and expenditure, providing better knowledge of a species’ lifestyle. The conventional trend has been to explore group-level time-activity budgets; however, individuals may also vary in their time-activity budgets (e.g., one individual foraging more than another), with the influencing mechanisms still poorly understood. We propose that animal personality, a behavioural and cognitive profile that makes one individual different from another, may explain why individuals vary in their time-activity budgets. We used a multi-method approach comprised of behavioural observations and experiments to assess the personality traits of lion-tailed macaques. The observed traits were used to predict individual time-activity budgets, broadly categorised into food-related, active, and resting behaviours. We then discuss the significance of this novel approach in light of lion-tailed macaque ecology, conservation, and welfare. Time-activity budget, i.e., how a population or an individual divides their day into various behaviours and activities, is an important ecological aspect. Existing research primarily focused on group-level time-activity budgets, while individual variations have only been reported recently. However, little is known about how consistent inter-individual differences or personalities influence time-activity budgets. We examined the personalities of lion-tailed macaques (Macaca silenus) and investigated their influence on individual time-activity budgets. The resulting personality traits, namely persistence, sociability, affiliation, and anxiety, were used to predict the three broad categories of the time-activity budget—food-related, active, and resting behaviours. We found that persistence and sociability positively predicted the time spent being active. Food-related behaviours were positively predicted by persistence, while anxiety was found to influence them negatively. The time spent resting was negatively predicted by persistence. We did not find an effect of affiliation on the time-activity budgets. We discuss these findings in light of the ecology of lion-tailed macaques. Our study highlights the importance of a novel approach that uses animal personality traits as predictors of individual time-activity budgets and offers insights regarding the use of personality assessments in conservation and welfare activities.

Slow improvement to the archiving quality of open datasets shared by researchers in ecology and evolution.

Many leading journals in ecology and evolution now mandate open data upon publication. Yet, there is very little oversight to ensure the completeness and reusability of archived datasets, and we currently have a poor understanding of the factors associated with high-quality data sharing. We assessed 362 open datasets linked to first- or senior-authored papers published by 100 principal investigators (PIs) in the fields of ecology and evolution over a period of 7 years to identify predictors of data completeness and reusability (data archiving quality). Datasets scored low on these metrics: 56.4% were complete and 45.9% were reusable. Data reusability, but not completeness, was slightly higher for more recently archived datasets and PIs with less seniority. Journal open data policy, PI gender and PI corresponding author status were unrelated to data archiving quality. However, PI identity explained a large proportion of the variance in data completeness (27.8%) and reusability (22.0%), indicating consistent inter-individual differences in data sharing practices by PIs across time and contexts. Several PIs consistently shared data of either high or low archiving quality, but most PIs were inconsistent in how well they shared. One explanation for the high intra-individual variation we observed is that PIs often conduct research through students and postdoctoral researchers, who may be responsible for the data collection, curation and archiving. Levels of data literacy vary among trainees and PIs may not regularly perform quality control over archived files. Our findings suggest that research data management training and culture within a PI's group are likely to be more important determinants of data archiving quality than other factors such as a journal's open data policy. Greater incentives and training for individual researchers at all career stages could improve data sharing practices and enhance data transparency and reusability.

Judgment Bias During Gestation in Domestic Pigs.

In humans and rats, changes in affect are known to occur during pregnancy, however it is unknown how gestation may influence mood in other non-human mammals. This study assessed changes in pigs' judgment bias as a measure of affective state throughout gestation. Pigs were trained to complete a spatial judgment bias task with reference to positive and negative locations. We tested gilts before mating, and during early and late gestation, by assessing their responses to ambiguous probe locations. Pigs responded increasingly negatively to ambiguous probes as gestation progressed and there were consistent inter-individual differences in baseline optimism. This suggests that the pigs' affective state may be altered during gestation, although as a non-pregnant control group was not tested, an effect of learning cannot be ruled out. These results suggest that judgment bias is altered during gestation in domestic pigs, consequently raising novel welfare considerations for captive multiparous species.

Personality and social environment predict cognitive performance in common marmosets (Callithrix jacchus).

Consistent inter-individual variation in cognition has been increasingly explored in recent years in terms of its patterns, causes and consequences. One of its possible causes are consistent inter-individual differences in behaviour, also referred to as animal personalities, which are shaped by both the physical and the social environment. The latter is particularly relevant for group-living species like common marmosets (Callithrix jacchus), apt learners that display substantial variation in both their personality and cognitive performance, yet no study to date has interlinked these with marmosets’ social environment. Here we investigated (i) consistency of learning speed, and (ii) whether the PCA-derived personality traits Exploration-Avoidance and Boldness-Shyness as well as the social environment (i.e., family group membership) are linked with marmosets’ speed of learning. We tested 22 individuals in series of personality and learning-focused cognitive tests, including simple motor tasks and discrimination learning tasks. We found that these marmosets showed significant inter-individual consistency in learning across the different tasks, and that females learned faster than males. Further, bolder individuals, and particularly those belonging to certain family groups, learned faster. These findings indicate that both personality and social environment affect learning speed in marmosets and could be important factors driving individual variation in cognition.

Social diffusion of new foraging techniques in the Southern ground-hornbill (Bucorvus leadbeateri).

Social learning during foraging has been found in a wide range of animals, including numerous bird species. Still, the mechanisms underlying this cognitive capacity remain largely unstudied and the use of divergent methods limits our understanding of their taxonomic distribution. Using an ecologically relevant design, the open diffusion experiment, we tested whether 11 Southern ground-hornbills (Bucorvus leadbeateri) were able to show imitation on the two-action task. Three experimental groups were created. In the slide and pull group, subjects ('observers') watched a trained conspecific ('demonstrator') opening a box using a specific technique. Naïve individuals from the control group, however, did not receive a social demonstration. All birds of the slide and pull group succeeded in opening the box, whereas all subjects of the control group failed the task. We found consistent inter-individual differences among some observers, with only two birds (one in each group) using the same technique and part of the box contacted by the demonstrator. Our results suggest that at least fine-tune enhancement underlies behavioural diffusion in this novel model species, which provides new research opportunities with direct implications for conservation.

Effects of Personality Traits on the Food-Scratching Behaviour and Food Intake of Japanese Quail (Coturnix japonica).

Simple SummaryScratching can help animals find more buried food and thus is an important food-searching behaviour for ground-feeding birds such as gamebirds. Due to the existence of animal personality, individuals within a population may exhibit different food-scratching patterns. This study tested the impacts of personality traits (i.e., boldness and exploration) on food-scratching behaviour and food intake of Japanese quail (Coturnix japonica). We found that boldness and exploration were repeatable, respectively, and were correlated. When entering a food patch, proactive (i.e., bolder and more explorative) quails scratched for food earlier and more frequently with a longer time. Frequent and longer food-scratching may motivate longer foraging time in proactive quails which can get more food intake. The correlation between personality and food intake was sex dependent. Proactive females had more food intake during the first half of the foraging process and the correlation became weak as time went on. The pattern was opposite in males. In conclusion, our study suggests that personality traits have significant effects on animals’ food-searching strategies which may be correlated with their foraging success and fitness.Overall foraging success and ultimate fitness of an individual animal is highly dependent on their food-searching strategies, which are the focus of foraging theory. Considering the consistent inter-individual behavioural differences, personality may have a fundamental impact on animal food-scratching behaviour, which remains largely unknown. In this study, we aimed to investigate how personality traits (i.e., boldness and exploration) affect the food-scratching behaviour and food intake of the domestic Japanese quail Coturnix japonica during the foraging process. The quails exhibited significant repeatability in boldness and exploration, which also constituted a behavioural syndrome. More proactive, that is, bolder and more explorative, individuals scratched the ground more frequently for food and began scratching earlier in a patch. Individuals that scratched more frequently had a longer foraging time and a higher food intake. The correlation between personality traits and temporary food intake during every 2 min varied over time and was sex dependent, with females exhibiting a positive correlation during the first half of the foraging stage and males after the initial stage. These findings suggest that personality traits affect the food-scratching behaviour and, thus, the food intake of quails. Our study provides insights into the impact of personality traits on animal’s foraging behaviour by influencing their food-searching strategies.

Studying Antillean manatees’ (Trichechus manatus manatus) temperament in zoological parks: exploration of boldness, sociality and reactivity to humans

Temperament, i.e. consistent inter-individual differences in behavior, has been documented in many species. Especially, boldness-shyness continuum is the ecologically relevant trait most frequently used to describe temperament among species. Reactions along the bold-shy axis demonstrate an organism’s ability to cope with environmental conditions and reflect its response to environmental stress. Our study investigated the existence of boldness-shyness continuum in 16 captive Antillean manatees (Trichechus manatus manatus) housed in 2 zoological parks. We first aimed to establish a valid methodology to measure boldness and shyness using behavioral tests. Additionally, our study explored 2 other potential temperament dimensions: sociality and reactivity to humans, and their association with boldness. Finally, since animals were kept under human care, we investigated human perception on manatees’ temperament. We conducted behavioral tests i.e. novelty tests involving novel objects, a novel sound, and an unfamiliar human. We recorded behaviors related to exploration as well as social interactions (i.e. physical proximity, social contacts) and we assessed manatees’ reactivity to humans through a participation score during hand-feeding sessions. Finally, we collected keepers’ subjective impression about individuals’ temperament through traits rating method. To our knowledge, this is the first paper that tested manatees’ reactions to different novel stimuli in order to determine their temperament. Our results first allowed us to identify the most relevant stimuli and variables to measure boldness and to classify individuals as bolder or shier. Individuals characterized as bolder displayed significantly more approach, longer approach and more contact toward stimuli than shier individuals. Moreover, boldness score was significantly associated with physical proximity with conspecifics, as well as with the participation score to hand-feeding sessions in one zoo. Finally, surveys completed by keepers were reliable and positive associations were found between rated items and behavioral observations. These findings support that as many species, manatees demonstrate boldness-shyness traits. More investigations on a larger sample size are needed to explore manatees’ temperament dimensions, and how these traits could modulate their interactions with their conspecifics and their environment. Studying manatees’ temperament could help improving the management and welfare of the species under human care. It could also facilitate their management in wildlife reserves by understanding manatees’ movements and evaluate the anthropogenic impact on wild populations.

Agonistic experience during development establishes inter-individual differences in approach-avoidance behaviour of crickets

Members of numerous animal species show consistent inter-individual differences in behaviours, but the forces generating animal “personality” or individuality remain unclear. We show that experiences gathered solely from social conflict can establish consistent differences in the decision of male crickets to approach or avoid a stimulus directed at one antenna. Adults isolated for 48 h from a colony already exhibit behavioural differences. Prior to staging a single dyadic contest, prospective winners approached the stimulus whereas prospective losers turned away, as they did also after fighting. In contrast, adults raised as nymphs with adult males present but isolated from them as last instar nymphs, all showed avoidance. Furthermore, adults raised without prior adult contact, showed no preferred directional response. However, following a single fight, winners from both these groups showed approach and losers avoidance, but this difference lasted only one day. In contrast, after 6 successive wins or defeats, the different directional responses of multiple winners and losers remained consistent for at least 6 days. Correlation analysis revealed examples of consistent inter-individual differences in the direction and magnitude of turning responses, which also correlated with individual aggressiveness and motility. Together our data reveal that social subjugation, or lack thereof, during post-embryonic and early adult development forges individuality and supports the notion of a proactive–reactive syndrome in crickets.

Aggressive males are more attractive to females and more likely to win contests in jumping spiders

Consistent interindividual differences in behaviour (i.e. personality) and intraindividual variability in behaviour (higher intraindividual variability means lower behavioural predictability) are common across animal taxa. However, how personality and behavioural predictability of males and females influence female mate choice and male–male competition remains poorly understood. Here, we investigated this in the jade jumping spider, Siler semiglaucus. After assessing the level of aggression (an individual's average aggression) and aggression predictability (the variability around average aggression within an individual) of both S. semiglaucus males and females, we performed female mate choice trials to test whether aggression and aggression predictability in females, males or both would affect female mate choice. We also conducted male contest trials to test whether male aggression or aggression predictability would influence the outcomes of male contests. We found that both females and males showed consistent interindividual differences in aggression, and aggressive spiders were more predictable than less aggressive ones. Despite a positive correlation between aggression and predictability, male aggression predicted female mate choice better than aggression predictability. Females showed a directional preference for aggressive males over docile males regardless of female aggression or male aggression predictability. Predictable aggressive males were also more likely to win contests. Our results suggest that both female mate choice and male–male competition favour males with high aggression, and thus total sexual selection that acts on male aggression may be reinforcing. These findings also highlight that male S. semiglaucus with a higher level of aggression may have better reproductive performance.